In some high energy ion implantation technology, the linewidth of features has required to be as small as 0.15 um, and the height of photoresist used to etch has reached a thickness of 4 um, so aspect ratio has been greater than 20.
Since the above requirements have exceeded traditional lithography processes limits, a new lithography process is needed. Generally, a tri-layer photoresist process is employed in the above high aspect ratio requirement, which is comprised of an injection barrier layer, a hard mask layer, and a photoresist layer. The pattern on the photoresist layer is successively transferred to the hard mask layer and the injection barrier layer by utilizing selection ratio change of etching among the photoresist layer, the hard mask layer and the injection barrier layer.
However, in the above prior art, when the ratio between the thickness of the injection barrier layer and the critical dimension is excessively large, a cone-shaped or a bowling-shaped photoresist etching morphology trends to be formed after etching, which will cause the insufficient of the selection ratio or larger critical dimension.
In addition, if the injection barrier layer is too thick, the edge of the hard mask layer is prone to be over etched, which will cause burrs on the pattern edge of the injection barrier layer.
The above-mentioned problems will affect the effect of implantation and the device performances.
Therefore, how to solve the problem of the cone-shaped or the bowling-shaped photoresist etching morphology existed in the prior art, and to solve the problem of the generated burr on the pattern edge, so as to make the implanted ions be well distributed according to the design of the device, becomes an urgent issue to be solved.